Central Americans who arrive at U.S. border crossings seeking asylum in the United States will have to wait in Mexico while their claims are processed under sweeping new measures the Trump administration is preparing to implement, according to internal planning documents and three Department of Homeland Security officials familiar with the initiative.

According to DHS memos obtained by The Washington Post on Wednesday, Central American asylum seekers who cannot establish a “reasonable fear” of persecution in Mexico will not be allowed to enter the United States and would be turned around at the border.

The plan, called “Remain in Mexico,” amounts to a major break with current screening procedures, which generally allow those who establish a fear of return to their home countries to avoid immediate deportation and remain in the United States until they can get a hearing with an immigration judge. Trump despises this system, which he calls “catch and release,” and has vowed to end it.

Among the thousands of Central American migrants traveling by caravan across Mexico, many hope to apply for asylum due to threats of gang violence or other persecution in their home countries. They had expected to be able to stay in the United States while their claims move through immigration court. The new rules would disrupt those plans, and the hopes of other Central Americans who seek asylum in the United States each year.

Trump remains furious about the caravan and the legal setbacks his administration has suffered in federal court, demanding hard-line policy ideas from aides. Senior adviser Stephen Miller has pushed to implement the Remain in Mexico plan immediately, though other senior officials have expressed concern about implementing it amid sensitive negotiations with the Mexican government, according to two DHS officials and a White House adviser with knowledge of the plan, which was discussed at the White House on Tuesday, people familiar with the matter said.

The White House did not immediately respond to a request for comment.

According to the administration’s new plan, if a migrant does not specifically fear persecution in Mexico, that is where they will stay. U.S. Citizenship and Immigration Services is sending teams of asylum officers from field offices in San Francisco, Washington, and Los Angeles to the ports of entry in the San Diego area to implement the new screening procedures, according to a USCIS official.

To cross into the United States, asylum seekers would have to meet a relatively higher bar in the screening procedure to establish that their fears of being in Mexico are enough to require immediate admission, the documents say.

“If you are determined to have a reasonable fear of remaining in Mexico, you will be permitted to remain in the United States while you await your hearing before an immigration judge,” the asylum officers will now tell those who arrive seeking humanitarian refuge, according to the DHS memos. “If you are not determined to have a reasonable fear of remaining in Mexico, you will remain in Mexico.”

Mexican border cities are among the most violent in the country, as drug cartels battle over access to smuggling routes into the United States. In the state of Baja California, which includes Tijuana, the State Department warns that “criminal activity and violence, including homicide, remain a primary concern throughout the state.”

The new rules will take effect as soon as Friday, according to two DHS officials familiar with the plans.

Katie Waldman, a spokeswoman for DHS, issued a statement late Wednesday saying there are no immediate plans to implement these new measures.

“The President has made clear — every single legal option is on the table to secure our nation and to deal with the flood of illegal immigrants at our borders,” the statement says. “DHS is not implementing such a new enforcement program this week. Reporting on policies that do not exist creates uncertainty and confusion along our borders and has a negative real world impact. We will ensure — as always — that any new program or policy will comply with humanitarian obligations, uphold our national security and sovereignty, and is implemented with notice to the public and well coordinated with partners.”

A Mexican official, speaking on the condition of anonymity, said that current Mexican immigration law does not allow those seeking asylum in another country to stay in Mexico.

On Dec. 1, a new Mexican president, Andrés Manuel López Obrador, will be sworn in, and it’s also unclear whether his transition team was consulted on the new asylum screening procedures.

The possibility that thousands of U.S.-bound asylum seekers would have to wait in Mexico for months, even years, could produce a significant financial burden for the government there, especially if the migrants remain in camps and shelters on a long-term basis.

There are currently 6,000 migrants in the Tijuana area, many of them camped at a baseball field along the border, seeking to enter the United States. Several thousand more are en route to the city as part of caravan groups, according to Homeland Security estimates.

Last week, BuzzFeed News reported that U.S. and Mexican officials were discussing such a plan.

Mexico also appears to be taking a less-permissive attitude toward the new migrant caravans now entering the country.

Authorities detained more than 200 people, or nearly all of the latest caravan, who recently crossed Mexico’s southern border on their way to the United States. This is at least the fourth large group of migrants to cross into Mexico and attempt to walk to the U.S. border. They were picked up not long after crossing. The vast majority of the migrants were from El Salvador, according to Mexico’s National Immigration Institute.

After the first caravan this fall entered Mexico, President Enrique Peña Nieto’s administration offered migrants the chance to live and work in Mexico as long as they stayed in the southern states of Chiapas and Oaxaca. Most chose not to accept this deal, because they wanted to travel to the United States.